Mechanisms of inhibition of Rhizobium etli pyruvate carboxylase by L-Aspartate

Abstract

L-aspartate is a regulatory feedback inhibitor of the biotin-dependent enzyme pyruvate carboxylase in response to increased levels of tricarboxylic acid cycle intermediates. Detailed studies of L-aspartate inhibition of pyruvate carboxylase have been mainly confined to eukaryotic microbial enzymes, and aspects of its mode of action remain unclear. Here we examine its inhibition of the bacterial enzyme Rhizobium etli pyruvate carboxylase. Kinetic studies demonstrated that L-aspartate binds to the enzyme cooperatively and inhibits the enzyme competitively with respect to acetyl-CoA. L-aspartate also inhibits activation of the enzyme by MgTNP-ATP. The action of L-aspartate was not confined to inhibition of acetyl-CoA binding, because the acetyl-CoA-independent activity of the enzyme was also inhibited by increasing concentrations of L-aspartate. This inhibition of acetyl-CoA-independent activity was demonstrated to be focused in the biotin carboxylation domain of the enzyme, and it had no effect on the oxamate-induced oxaloacetate decarboxylation reaction that occurs in the carboxyl transferase domain. L-aspartate was shown to competitively inhibit bicarbonate-dependent MgATP cleavage with respect to MgATP but also probably inhibits carboxybiotin formation and/or translocation of the carboxybiotin to the site of pyruvate carboxylation. Unlike acetyl-CoA, L-aspartate has no effect on the coupling between MgATP cleavage and oxaloacetate formation. The results suggest that the three allosteric effector sites (acetyl-CoA, MgTNP-ATP, and L-aspartate) are spatially distinct but connected by a network of allosteric interactions.